Abscisic acid(hereinafter referred to as "ABA") represented as structural formula below, is one of plant growth hormones natually occurring in plants; and, it is a causal substance of abscission and dormancy in plants. In addition to this, biological activity of ABA is greatly diverse, including inhibition of germination and growth, inhibition of .alpha.-amylase induction, inhibition of transpiration, and induction of stomatal closure. ABA has also been found to be a substance inducing anti-stress responses, in a view of the fact that the level of ABA is elevated in plants responding to environmental stresses such as drought and cold temperature. ##STR2##
Recently, the structure-activity relationship of ABA has been actively studied by several groups of workers. It is summarized as follows: To possess biological activity, the side chain structure should have carboxylic group, and 2-cis, 4-trans configuration, and methyl group at C-3. In terms of ring system, methyl group and double bond at C-2' are essential. Further, carbon at C-4' should be ketone or be able to be oxidized to become ketone. However, all of ABA analogs having alcohol group instead of carboxylic group, with different configurations, and having no methyl substitution or a longer alkyl group, are less inactive than natural ABA.
Natural or intact ABA, however, has not been developed into a practical plant growth regulator, grounded on the following reasons:
first, natural ABA does not show stable and steady activity, since double bond in side chain is easily isomerized to produce inactivated form; and, PA1 secondly, the level of natural ABA cannot be maintained in an effective amount, since it is easily metabolized to biologically inactive phaseic acid. PA1 R is hydroxymethyl, aldehyde, C.sub.1-2 alkoxycarbonyl or carboxyl group.
In this connection, various modifications were made to the structure of ABA to solve the technical task in the art as follows:
Todoroki et al disclose a process for preparing ABA in which C-8' or C-9' methyl group is replaced with a methoxy group; the process, however, requires complex and vigorous reaction condition employing reactant hard to handle such as bromine. Moreover, since isomeric mixture of the methoxy-ABA and biologically inactive 2-trans-4-trans isomer is produced during the hydrolysis step of reaction intermediate, laborous separation step is essentially required to obtain the desired ABA in isolated form. In addition, (+)-9'-methoxy-ABA is more effective than (+)-8'-methoxy-ABA, which is expected to be an antimetabolic analog, suggesting that methoxy group itself might affect the activity(see: Y. Todoroki et al, Biosci. Biotech. Biochem., 58(4):707-715(1994)).
Kim et al describe a process for preparing ABA whose side chain is replaced with phenyl group; the process, however, accompanies highly complicate reaction steps. Further, the biological activity of ABA derivative prepared therefrom, is much lower than that of the natural ABA(see: B. T. Kim et al, Biosci. Biotech. Biochem., 56(4):624-629(1992)).
Nelson et al teach a process for preparing 7'-hydroxy abscisic acid; the process, however, not only requires expensive chemical reagents such as t-butyldimethylsilyl chloride, imidazole and tetrabutylaluminum fluoride, but also accompanies complex silylation and desilylation reaction steps. In addition, selective reduction of aldehyde group of .alpha.,.beta.-unsaturated ketoadlehyde intermediate is performed only under specific conditions(see: L. A. K. Nelson et al, Tetrahedron, 47(20/21):3259-3270(1991)).
Rose et al demonstrate a process for preparing 7,7'-difluorinated ABA by the fluorination of ketoaldehyde with diethylaminosulphur trifluoride("DAST"); the process, however, also requires expensive reactants such as DAST, but also accompanies arduous fluornation step.
Under the circumstances, these prior art ABA derivatives have proven less satisfactory in the sense that they cannot be prepared in a practical manner and do not provide desired biological activity; and, therefore, there is a need in the art for the development of ABA derivative which possesses stable and steady biological activity and process for preparing same in a simple and economical manner.